1. Field of the Invention
The invention relates to a method and apparatus for identifying an operational phase of a motor phase winding and controlling energization of the phase winding.
2. Description of Related Art
The control of a switched reluctance machine (SRM) is highly dependent on the absolute position of its rotor. A detailed description of this relational dependency is provided in “Switched Reluctance Motor Drives,” R. Krishnan, CRC Press, June 2001. The position of the rotor can be measured using an encoder, a resolver, or optical switches with magnet wheels. A solution by any of the above means is expensive and, therefore, not applicable and desirable in low performance, high volume, and low-cost applications.
Another way to obtain the absolute rotor position is by way of estimating it indirectly through measurement and processing of current or voltage signals. The basis for such a method lies in the fact that a phase winding of the SRM has a distinct and unique three-dimensional relationship among its three key variables of inductance/flux linkages, excitation phase current, and rotor position. When values of two of the three variables are known, the third variable can be deduced from prior knowledge of the three-dimensional relationship. Currents can be measured easily and inexpensively and flux linkages can be computed (provided the voltages of the windings are available), thus making the rotor position extraction an easy and simple matter.
Voltage measurements are not always inexpensive, clean, and free of switching noise. Thus, implementing a method of measuring voltage values to derive the third variable often presents a challenge in practice. Also, the way in which the method is implemented determines the estimation accuracy, the motor drive's speed range, implementation cost, and the robustness of the implementation. A method to inexpensively sense, measure, or estimate the absolute rotor position, directly or indirectly, remains a significant research and development challenge.
In “Switched Reluctance Motor Drives,” Krishnan categorizes various methods of rotor position estimation and identifies their respective algorithms, implementations, merits, and demerits. Some related art methods are described below.
Watkins discloses in U.S. Pat. No. 5,793,179 using a freewheeling current to estimate rotor position. The relationship between the flux-linkage and the current in an SRM can be expressed as:
                                             λ            =                                          i                ·                L                            ⁢                                                          ⁢              …                                ⁢                                                                                                        ⅆ              λ                                      ⅆ              θ                                =                                    i              ·                                                ⅆ                  L                                                  ⅆ                  θ                                                      +                                          L                ·                                                      ⅆ                    i                                                        ⅆ                    θ                                                              ⁢                                                          ⁢              …                                                                                              ⅆ              i                                      ⅆ              θ                                =                                                    1                L                            ⁡                              [                                                                            ⅆ                      λ                                                              ⅆ                      θ                                                        -                                      i                    ·                                                                  ⅆ                        L                                                                    ⅆ                        θ                                                                                            ]                                      ⁢                                                  ⁢            …                              where i is the current, L is the inductance, λ is the flux linkage of a stator phase, and θ is the rotor position.
During freewheeling, the right-hand side of the second equation can be simplified, since the voltage across the phase winding will be low and, therefore, the rate of change of flux is negligible. This gives rise to the relationship:
            ⅆ      i              ⅆ      θ        =                    1        L            ⁡              [                              -            i                    ·                                    ⅆ              L                                      ⅆ              θ                                      ]              ⁢                  ⁢    …  
For a small Δi≈α[−iΔL], where α=(1/L) . . . .
Watkins' method uses the fact that at the fully aligned position, of a set of rotor poles with respect to a set of stator poles, the rate of change of inductance with angle is essentially zero, irrespective of the level of phase current. Thus, according to the equation above, if Δi=0, the rotor position is centered on the aligned position, where the rate of change of inductance is zero. If Δi≠0, then its sign will give an indication of the operating point of the SRM, indicating whether the SRM is operating in a positive or negative inductance slope region.
The advantage of Watkins' method is that it is fairly simple to deploy. The disadvantage of the method being that it requires the machine to be rotating, with current in at least one phase winding. Therefore, it requires a separate startup procedure and device.
Liu et al. disclose in U.S. Pat. No. 6,107,772 using the rise time of currents to detect rotor position. This method is based on the principle that current rise times in a motor phase coil are directly proportional to the inductance in the phase coil. The inductance in the phase coil is indicative of the rotor position and, hence, the rise times give an indication of the rotor position. The rise time is compared to a desired current rise time, which is indicative of an in-phase relationship between the conduction interval and rotor position. Therefore, the rise time can be used to generate control signals that govern commutation instances.
MacMinn et al. disclose in U.S. Pat. No. 4,642,543 an SRM drive system that includes a state sequencing mechanism that is coupled to a phase sequencer to achieve position-based operation of a motor, without using a position sensor. A current sensor provides the state sequencing mechanism with the average current going through the motor windings, and the state sequencer generates the starting sequences based on the current information. The phase sequencer determines the phase to be energized, based on information received from the state sequencing mechanism. The start-up state sequence may be modeled by a state diagram having an align state, a start state, a check state, and a ramp state. From the align state, a transition is made to the start state, where an attempt to start the motor is made. Thereafter, a transition to the check state is effected and a determination of the average direct current (dc), Idc, in the motor windings is made. If Idc rises above a reference value, Iref, the start-up state sequence is reset to the align state, indicating the start-up sequence has to be re-tried. If Idc is less than Iref, the start-up sequence proceeds to the ramp state, which means that the start-up sequence is successful. Thereafter, the phase sequencer takes the motor to the commanded speed.
Konecny discloses in U.S. Pat. No. 5,015,939 a motor control circuit for an SRM that includes a rotor and a plurality of stator coils. A driver circuit for energizing the stator coils in sequence includes a current sensing resistor, for detecting a signal proportional to the current in the coils, and a slope detector for detecting the rate of change of the current signal with respect to time, over predetermined time intervals during which the voltage in the coils is dominated by a term representing back EMF. A variable oscillator is responsive to an integrated output of the slope detector for generating a timing signal for the driver circuit. When the voltage across the coils is dominated by the back EMF term, the current should be substantially constant. But when the rotor is out of phase with the driver circuit, the current signal during this time has a substantially linear slope, which is either positive or negative depending upon whether the rotor is leading or lagging the driver circuit.
Another method that uses active phase current sensing has been proposed by Sood et al. in U.S. Pat. No. 5,420,492 and Marcinkiewicz et al. in U.S. Pat. No. 5,457,375. This method uses the dc bus current waveform to gather information regarding commutation instants for the motor. The method uses a processor to evaluate the slope and amplitude of the dc bus current, compares this waveform to a desired waveform, and then adjusts the commutation frequency accordingly. Prior knowledge of the waveform is required and has to be stored in memory for the method to work. Marcinkiewicz improves upon Sood's method by introducing an algorithm that takes into account the ripple content.
MacMinn et. al. disclose in U.S. Pat. No. 4,959,596 using an inductance sensing method, where voltage sensing pulses are applied to an inactive phase. The change in phase current resulting from the sensing pulses is inversely proportional to the instantaneous value of the phase inductance. A proper commutation time is determined by comparing this change in phase current to a threshold current, thus synchronizing phase excitation to the rotor position. Phase excitation is advanced or retarded by decreasing or increasing the threshold, respectively.
External signal injection into an inactive phase is used by Green, in U.S. Pat. No. 6,291,949, to achieve sensorless operation of a polyphase SRM. In the chopping mode, diagnostic pulses of predetermined flux linkage are injected into an idle phase. In the single-pulse mode, position prediction is performed using an active phase. A method of starting the machine uses diagnostic pulses in two phases to provide a unique value for position, allowing the drive to start or re-start under full torque. In another variation of this method, Green discloses in U.S. Pat. No. 6,351,094 injecting voltage pulses into an idle period of each phase winding in a chopping mode. Both of these methods are suitable for polyphase machines. An attempt is made to reduce acoustic noise by varying the frequency at which the pulses are injected according to the rotor speed.
Mayes discloses in U.S. Pat. No. 6,396,237 using a method based on injecting a diagnostic pulse, having a predetermined maximum value of current or flux linkage, into an inactive phase winding. The current or the flux linkage is measured at the end of the pulse and at the start before the pulse is injected. A value for the current or flux-linkage, due to injection of the diagnostic pulse, from the difference between the total current and the main current or flux-linkage is determined and then used to infer the rotor position.
MacMinn discloses in U.S. Pat. No. 4,772,839 an indirect position sensor that applies short duration low-level sensing pulses to two un-energized phases. A change in the phase is sensed to produce two values of estimated position values, for each of the un-energized phases. One of the two values corresponding to each un-energized phase is shifted by a value, which is equal to the phase displacement between the two un-energized phases. The two values corresponding to each un-energized phase are used to give an estimate of a rotor position. If one of the initially inactive phase becomes energized, then an extrapolation algorithm is used to estimate the angle, until there are two un-energized phases available to use the above estimation technique. This technique is only usable for polyphase machines.
Lyons et al. disclose in U.S. Pat. No. 5,525,886 a method of estimating rotor position by applying relatively high frequency, short duration, probe pulses to at least two inactive phases. The voltage and the currents in these phases are measured and used to compute an estimate of the rotor and stator flux linkage in each of the phases. This estimate is fed into a model to determine the rotor position. The model can be a simple look-up table that has the rotor position corresponding to the potential flux values. The disadvantage of the method is that a look-up table has to be in place for this method to work. Also, measuring the voltages requires an expensive transducer part, which is unacceptable in many applications.
Kalpathi discloses in U.S. Pat. No. 5,786,681 using active phase coil inductance sensing to determine when to switch conduction to the next phase. The circuit has an asymmetric converter bridge and two switches per phase and operates as follows. First, both switches are turned on to initiate the conduction phase. Then, the bottom switch is kept closed while the top switch is switched on and off to produce local maximum and minimum currents. The amounts of time taken to reach each pair of local maxima and minima are recorded and their ratio is compared to a predetermined ratio to determine the commutation signal.
Ray discloses in U.S. Pat. No. 5,467,025 measuring the current and flux in an active phase, at a predicted rotor position value, and comparing the measured values to reference values. The angular error between the compared values identifies the predicted rotor position for the next cycle. The advantage of this method is that it requires the flux and the current only once every conducting cycle, unlike other flux linkage methods that compare the flux to a reference value continuously to determine the commutation time. But the disadvantage of this method is that it requires flux linkages, which in turn require both voltage and current sensing, making it difficult to implement compactly and with low cost.
A current gradient technique uses the fact that if the pulse width modulation (PWM) duty cycle is kept constant during one conduction period of a phase, then the slope of the current with respect to rotor position increases initially. When the inductance starts to rise, the current-slope decreases. Gabriel et al. disclose such a technique in IEEE Transactions on Industry Applications, Vol. 34, No. 4, pp. 832-834, July/August 1998. The limitations of this method are (1) the machine phases are triggered with an advance angle and (2) a separate starting mechanism needs to be in place to have full range of operation. The advantage of the method is that complex circuitry is not required to determine rotor position.
Holling et al. and Lim disclose in U.S. Pat. Nos. 5,600,218 and 5,589,751, respectively, using the current gradient technique. Their algorithms compare a differentiated, normalized, current waveform to a predetermined threshold. The result of the comparison can then be used to detect the commutation point.
Holling discloses using the algorithm for a brushless dc motor, though this use may be extended to a variable reluctance motor.
Another technique known in the related art is a flux linkage based method that provides hesitation free starting. A common drawback of position sensorless techniques is the hesitation that occurs while starting the SRM.
Bu and Xu disclose in “Eliminating Starting Hesitation for Reliable Sensorless Control of Switched Reluctance Motors,” Conference Record, IEEE Industry Applications Society, pp. 693-700, October 1998, a method for self starting without hesitation. All phases are excited for a short period and phase currents are measured. The phase with the second highest current is chosen to identify the rotor position. The drawback with this method is that a voltage transducer is needed in addition to current sensors.
Observer based estimation methods use model reference adaptive control (MRAC) techniques, simple observer/estimation techniques or a full observer technique. Lumsdaine and Lang disclose simple observer/estimation techniques in “State Observers for Variable-Reluctance Motors,” IEEE Transactions on Industrial Electronics, Vol. 37., No. 2, 1990, pp. 133-142. Gentili and Nicosia disclose a full observer technique in “Observer Based Control System Design for Switched Reluctance Motors,” Proceedings of the 34th Conference on Decision and Control, December 1995, pp. 115-120. Elmas and Zelaya-De La Parra also disclose a full observer technique in “Application of a Full Order Extended Luenberger Observer for a Position Sensorless Operation of a Switched Reluctance Motor Drive,” IEEE Proceedings on Control Theory Applications, Vol. 143, No. 5, 1996, pp. 401-408.
These methods are all computationally intensive and, hence, are not viable industrial solutions for high volume commercial applications. Also, most of the current research is incomplete. Furthermore, the issue of self-starting has not been addressed in any of the studies. These methods are also discussed in “Switched Reluctance Motor Drives,” R. Krishnan, CRC Press, June 2001.
Lyons et al. disclose in U.S. Pat. No. 5,097,190 a model based approach. This approach uses a multi-phase lumped parameter model of an SRM. The method uses a sensor to sense flux. The measured flux is used to look up a corresponding position in a ROM or a microprocessor look-up table. The drawback of this method, apart from its complexity, is that it utilizes a microprocessor or look-up table. Also, this method is not suitable at very high speeds, as the look-up table and the capability of the analog-to-digital converter limits its operation.
A more involved model has been developed by the present inventors where the flux linkages of all phases are measured, to indirectly determine the rotor position. In this application, the motor is modeled as a network driven by a magnetomotive force. The flux generated in the stator poles is determined, and from the phase current and flux measurements, the reluctance of each phase is determined. The phase reluctance provides an indication of the rotor position with respect to the aligned position. The model takes into account the multi-phase saturation, leakage, and mutual coupling effects. The disadvantage of this method is the expense incurred in measurement of flux.
Branecky discloses in U.S. Pat. No. 6,153,956 a method using the same principle of flux level control along with a look-up table. The difference lies in the fact that Branecky uses a switch to account for various waveforms of currents, namely flat current waveforms or other waveforms. This relationship is also reflected in the look-up table on the microprocessor side. The key disadvantage is in identifying the nature of current waveforms. Also, limiting set of waveforms to make the implementation easier may compromise accuracy of the rotor position estimation.
Hedlund et al. disclose in U.S. Pat. No. 5,173,650 determining the actual flux value and comparing this flux value with a reference flux value, which is a predetermined non-linear function of current. The premise of the invention lies in the fact that for each angular position, there is a nonlinear relationship between magnetic flux and current in the windings of the stator pole pairs. The predetermined value of the flux corresponds to the angle at which the commutation between phases is desired. When the actual value exceeds or equals the reference value of the flux, commutation occurs. As with all the flux or flux linkage-based implementations, the flux or flux linkage estimation requires voltage sensing, which increases the sensing burden and thereby increases the complexity of the implementation.
Heglund discloses in U.S. Pat. No. 6,359,412 utilizing a relative angle estimation circuit, an angle combination circuit, and an estimator that includes a Kalman filter. Phase currents are measured in the relative angle estimator. The angle combination circuit combines the estimates to obtain an absolute angle, which eliminates ambiguities. The estimator uses the model of an SRM to further increase the accuracy of the estimated rotor position and velocity. This method uses modern control theory and complex circuitry that prohibits its use in high volume applications.
Morris discloses in U.S. Pat. No. 6,150,778 a method for sensorless operation of an SRM having at least one irregular pole. The irregularity may be provided by its geometric shape, material properties, or any other characteristic that can give rise to an irregular inductance profile. Voltage is sensed across one phase winding and compared to a reference voltage to generate an encoder output that is indicative of the rotor position. The encoder signal is then used to control the excitation of the phase.
In another variation of this method, the voltage across a pair of windings is measured. Due to the irregular shape of the rotor, the voltages across these two sets of windings, of the same phase, follow a set pattern and can be used for rotor position detection. To determine actual rotor position, an analog voltage ratio is determined. By mapping the voltage ratio versus rotor position (for one or more phases), the actual rotor position is looked up from a look-up table of rotor position data, using the computed voltage ratio. Since the method uses ratiometric data, the position information supplied is insensitive to supply voltage, winding temperature, nominal inductance, and other operating conditions. The advantages of this method are (1) it doesn't require conventional sensors, (2) it is insensitive to the supply voltage differences, in inductance, due to the manufacturing variations, and (3) it is low cost, easier to implement, and can be used in harsh environments. The disadvantage is that in case of ambiguous position information, information has to be present for more than one phase to remove the ambiguity. Also, this method requires a separate start-up procedure and is unsuitable for high speed operation.
MacCann discloses in U.S. Pat. Nos. 5,691,591 and 5,949,211 using auxiliary windings interspersed with the main winding. The controller is capable of individually energizing the main as well as the auxiliary windings, individually. A voltage sensor determines the voltage across an auxiliary winding when it is energized. The phase of the current in the auxiliary is measured and the difference between the two phases is used to provide a signal that is indicative of the position of the rotor.
Bartos et al. disclose in U.S. Pat. No. 5,256,923 a method that uses irregularities in either a rotor or stator pole. The algorithm revolves around the fact that one or more irregularities in either the stator or rotor poles, or both, create a distinct pattern in an otherwise standard inductance profile. The unique shape of the rotor causes a disturbance in the normal inductance profile. The frequency at which the disturbances occur in the inductance profile can be used to get speed information as well.
All reference material cited herein is hereby incorporated into this disclosure by reference.